In recent years, with rapid expansion of a compact-type electronic device such as a mobile phone, a notebook-type personal computer, demand of the non-aqueous electrolyte secondary battery, as a power source enabling charge-discharge, has been increasing rapidly. As the positive electrode active material for the non-aqueous electrolyte secondary battery, a lithium cobalt composite oxide represented by lithium cobaltate (LiCoO2), as well as the lithium nickel composite oxide represented by lithium nickelate (LiNiO2), a lithium manganese composite oxide represented by lithium manganate (LiMnO2) and the like have been widely used.
Cobalt used in lithium cobaltate is expensive due to scarce reserves, and thus has problems of unstable supply and large price fluctuation. In addition, in the case of using lithium cobaltate or the lithium nickel cobalt composite oxide obtained by its modification, there has been a problem of gradual destruction of a crystal structure caused by large change thereof in charging-discharging, resulting in decrease in discharge capacity.
Lithium manganate or lithium nickelate having manganese or nickel as a main component, which is relatively cheap, has been attracted attention in view of cost, however, lithium manganate has many practical problems as a battery, because of having very small charge-discharge capacity, as well as having very short charge-discharge cycle characteristics, which indicates battery lifetime in using as a battery.
On the other hand, lithium nickelate is expected as the positive electrode active material which is capable of producing battery with high energy density in low cost, because of showing larger charge-discharge capacity as compared with lithium cobaltate, however, had a defect of inferior heat stability in a charged state as compared with lithium cobaltate. That is, pure lithium nickelate has a problem in heat stability or charge-discharge cycle characteristics, and thus it was impossible to be used as a practical battery. This is because of having lower stability of a crystal structure in a charged state, as compared with lithium cobaltate.
Under such a circumstance, in order to provide a non-aqueous battery which is capable of decreasing change of a crystal structure in charge-discharge, dramatically increasing discharge capacity, as well as enhancing thermal stability, there has been proposed an invention for using, as a positive electrode material, LiaMbNicCOdOe (wherein M is at least one kind of a metal selected from the group consisting of Al, Mn, Sn, in, Fe, V, Cu, Mg, Ti, Zn, Mo; a, b, c, d and e are in a range of 0<a<1.3, 0.02≦b≦0.5, 0.02≦d/c+d≦0.9, 1.8<e<2.2; and further b+c+d=1) (Refer to pages 1 and 2 of PATENT LITERATURE 1).
In this invention, it is said that by configuring M of the positive electrode active material by at least one kind of a metal selected from the group consisting of Cu and Fe, thermal stability can be enhanced significantly, in the presence of an electrolytic solution after charging.
However, the above lithium nickel composite oxide has a problem of practically decreasing charge-discharge capacity, due to substitution of a part of nickel with other elements. In addition, there is a serious problem that ratio of discharge capacity relative to initial time charge capacity (initial time charge-discharge efficiency) decreases significantly, in the case where the lithium nickel composite oxide contains Al. In addition, such a problem has also been pointed out that, because of presence of lithium carbonate or lithium sulfate inside the positive electrode active material after synthesis, these lithium compounds generate gas by oxidative decomposition, when the positive electrode active material is charged under high temperature environment.
In recent years, the lithium nickel composite oxide has become to be used as the positive electrode active material also in a polymer-based battery which uses an aluminum laminate material or the like as an exterior packaging, however, in such a case, as described above, generation of gas by decomposition of lithium carbonate or the like in the lithium nickel composite oxide, during use of a battery, incurs dimensional defect, or significantly deteriorates battery performance. To eliminate such a problem, such an invention has been proposed that prevents generation of lithium carbonate inside the positive electrode active material, by adding natural water to the lithium composite oxide obtained by heat treatment, to attain a slurry concentration of 300 g/l, stirring and removing an unreacted lithium salt, so as to remove lithium carbonate in the lithium nickel composite oxide after synthesis (Refer to page 2 of PATENT LITERATURE 2).
However, this method increases Li+ ion concentration in water, which could re-precipitate lithium hydroxide or lithium carbonate after drying, as well as has a problem of easy deterioration of charge-discharge capacity.
To eliminate such a defect, there has been disclosed a method for obtaining the lithium nickel composite oxide by firing a mixture obtained by mixing raw materials by each predetermined amount, washing this with 500 ml or more of water, relative to 100 g of the relevant lithium composite oxide, dehydrating, and performing a series of the steps from water washing to dehydration within 4 hours, and then drying the dehydrated lithium nickel composite oxide, till residual moisture content, when measured at a measurement temperature of 250° C., using a Karl Fischer moisture meter, attains 800 ppm or less, in a constant temperature chamber having air atmosphere of 200° C. or higher, or vacuum atmosphere. In this way, lithium carbonate or lithium sulfate generating during synthesis of the relevant lithium composite oxide can be removed sufficiently, or re-crystallization of Li ions can be prevented as well. And, by using the positive electrode active material obtained in this way, it is said that the non-aqueous electrolyte secondary battery having not only enhanced charge-discharge efficiency but also gas generation suppressed even under high temperature environment (Refer to pages 2, 3 and 4 of PATENT LITERATURE 3).
However, by water washing the lithium nickel composite oxide at random, there are problems of giving unclear influence on change of specific surface area and enhancement of heat stability after water washing, and giving the case of low slurry concentration in water washing, or elution of a large quantity of lithium ions, or generation of structural change, that is change of a substance itself, caused by high temperature, in the case of limiting to the lithium nickel composite oxide, caused by high temperature processing after water washing.
To eliminate these problems, the present inventors have previously proposed a method for obtaining powder of the lithium nickel composite oxide having superior characteristics, by using (a) a step for preparing the nickel oxide by firing the nickel hydroxide or the nickel oxyhydroxide containing nickel as a main component, and at least one kind of an element selected from other transition metal element, the second group element and the thirteenth group element, as a minor component under air atmosphere at a specific temperature range; (b) a step for preparing fired powder represented by represented by the composition formula (1):LiNi1-aMaO2  (1)(wherein M represents at least one kind of an element selected from a transition metal element other than Ni, the second group element and the thirteenth group element; a satisfies 0.01≦a≦0.5), by mixing the nickel oxide and a lithium compound under oxygen atmosphere at a specific temperature range; and (c) a step for filtering and drying, after water washing the fired powder in water for specific period (Refer to pages 1 and 2 of PATENT LITERATURE 4).
One of the characteristics of this method is in adjustment of relation between water washing time (A), as a specific water washing time in the step (c), and a slurry concentration (B) of the lithium nickel composite oxide, within a range satisfying AB/40 (wherein A represents the water washing time indicated by a unit of minute; and B represents the slurry concentration indicated by a unit of g/L), and by adjustment of this water washing time, the positive electrode active material, having a true specific surface area obtained by washing off the impurities at the surface of fired powder, of 0.3 to 2.0 m2/g, can be obtained, resulting in large capacity, low price and superior heat stability, suitable as the non-aqueous electrolyte secondary battery.
In this way, we have come to accomplish a proposal satisfying to certain extent, as for a problem of obtaining the positive electrode active material for the non-aqueous electrolyte secondary battery having large capacity, low price and superior heat stability.
However, in recent years, a lithium ion non-aqueous electrolyte secondary battery has begun to be used in power tool applications such as electric tools other than a mobile phone, and mounting it onto a large current device has already been started, and thus rapid expansion of a market of the power-type non-aqueous electrolyte secondary battery as a power application is expected.
As a point to be put importance as performance of such a power-type non-aqueous electrolyte secondary battery, there is output characteristics other than conventionally required battery capacity and heat stability. Insufficient output characteristics of a battery generate a problem of inability of complete utilization of battery performance. As for output characteristics of a battery, inner resistance of the positive electrode active material has large influence, and small inner resistance is desired. The positive electrode active material obtained by the above production method proposed by the present applicants was not necessarily sufficient in view of inner resistance thereof, although it satisfies battery capacity and heat stability, as described above.
In view of the above circumstance, it has been required to obtain the positive electrode active material having inner resistance about 30% lower than a conventional level, as for output characteristics, while maintaining characteristics at least equivalent to or higher than the positive electrode active material obtained by the above production method, as for battery capacity and heat stability, and to realize the non-aqueous electrolyte secondary battery with high energy density, using the same.